1. Field of the Invention
This invention relates to an encoder and, in particular, to an encoder, such as a rotary encoder or a linear encoder, in which a luminous flux is caused to impinge upon a moving body having an optical scale formed by periodically arranging a plurality of diffraction gratings on the outer or the inner peripheral surface of a cylindrical body or on a planar surface, the luminous flux transmitted through this optical scale being utilized to detect movement information regarding the moving body.
2. Description of the Related Art
A rotary or linear photoelectric encoder has conventionally been used as a means for detecting the rotational speed, or variations in the rotational speed, of a rotating mechanism, or variations in the amount of movement of a linearly moving body. For example, such an encoder has been used in the floppy-disk driving mechanism of computer equipment, in an office machine, such as a printer, in a numerically controlled machine tool, in the capstan motor of a VTR, or in any other type of rotating mechanism such as a rotary drum.
FIG. 1 schematically shows the essential parts of a conventional rotary encoder utilizing Talbot interference.
Referring to FIG. 1, numeral 1 indicates a semiconductor laser which emits a coherent luminous flux having a wavelength .lambda.. Numeral 2 indicates a collimator lens system for converting a divergent luminous flux from the semiconductor laser 1 to an approximately parallel luminous flux. The semiconductor laser 1 and the collimator lens system 2 together form a light irradiation means LR. Numeral 3 indicates an optical scale having a translucent grating section consisting of a plurality of V-grooves formed on the inner peripheral surface of a cylindrical body at fixed intervals. The optical scale 3 rotates in the direction indicated by the arrow in FIG. 1.
The optical scale 3 is formed of a translucent optical material. Arranged to the side of the optical scale 3 opposite to the light irradiation means LR are three photodetectors 4a, 4b and 4c which form a light receiving means 4. The respective output terminals of these photodetectors are connected to a signal processing circuit 5, which includes a pulse counting circuit, a rotating direction judging circuit, a signal interpolation processing circuit, etc.
In the rotary encoder of FIG. 1, the luminous flux from the light irradiation means LR is caused to impinge upon an area 3a (a first scale) of the optical scale 3 to undergo optical modulation (diffraction), and is further caused to impinge upon another area 3b (a second scale) of the optical scale 3, where it undergoes further optical modulation (deflection) to be output from the optical scale 3 in a plurality of luminous fluxes, which are received by the light receiving means 4. By utilizing output signals from the light receiving means 4, rotational information regarding the optical scale 3 is detected.
FIG. 4 illustrates how the luminous flux from the first scale 3a of FIG. 1 impinges upon the second scale 3b.
The distance d between the gratings of the first and second scales 3a and 3b of the scale 3 as measured along the optical axis (i.e., the inner diameter of the scale 3) is set so as to satisfy the following equation: EQU d=N.sub.a .times.P.sup.2 /.lambda.(N.sub.a =1, 2, 3, . . . )
where P is the grating pitch and .lambda. is the wavelength of the luminous flux from the semiconductor laser 1.
Positioned at a distance of N.sub.a .times.P.sup.2 /.lambda. (N.sub.a =1, 2, 3, . . . ) from the first scale 3a is a Fourier image plane where a clear optical image is formed, which image has an intensity distribution of the same grating pitch as the grating of the first scale 3. At any other position in the direction of the optical axis, a reduction in intensity occurs at the same grating pitch as the grating of the first scale 3a, thus allowing the formation of an optical image having an intensity distribution of low contrast in which there is a conspicuous reduction in the high-frequency components.
Therefore, any change in the wavelength .lambda. of the luminous flux from the semiconductor laser resulting from a change in the environmental conditions, such as temperature, causes the Fourier-image-forming position of the grating of the first scale 3a to be offset relative to the grating surface of the second scale 3b. This causes a reduction in the S/N ratio of the detection signals detected by the light receiving means 4, resulting in a deterioration in the accuracy of detection of movement information regarding the optical scale 3.